New research shows why science hasn't been able to find a vaccine against HIV yet.

According to researchers from the Oregon Health and Science University, the problem involves finding an HIV strain that was just right to kick off an immune response in the body and not cause an illness.

Vaccines that protect against a disease are usually created using a weakened strain of the causal agent (bacteria or virus) or by using a dead strain of the same disease. Both of these methods aim at teaching the immune system how to fight the infection. So, the next time the causal agent attacks the body, the immune system knows how to defend itself.

"Efforts to develop a live attenuated virus are analogous to the tale of 'Goldilocks and the Three Bears,'" said Louis Picker, MD, associate director of the OHSU Vaccine and Gene Therapy Institute.

"The field was looking for a vaccine that was 'not too hot,' or 'not too cold,' but 'just right.' The problem was that it appears that weakening a virus to the level that is 'just right' is impossible. However, we thought that understanding the mechanism responsible for the protection afforded by the too-dangerous-for clinical-use attenuated vaccine would allow us to design a vaccine that would be both effective and safe," Picker added.

Vaccine for the deadly HIV too could be created this way. But, the counterpart of HIV, the SIV (simian immunodeficiency virus) failed to act as a good strain for the creation of a vaccine. A specific strain of SIV was strong enough to trigger an immune response and offer protection against AIDS but was also associated with full blown illness in some test animals (the vaccine was tested on monkeys). Researchers then had found that if they weakened the SIV strain any further they'd lose the strain's capacity to create an immune response.

In the present study, researchers found why the SIV strain wasn't effective in protecting the monkeys from AIDS. According to them, the virus wasn't persistent in getting the immune system ready for an HIV attack. The protection occurs due to the presence of anti-viral T cells in the body due to a persistent virus that have been slightly weakened.

Researchers have developed a persistent virus called cytomegalovirus (CMV) that acts as a vector or a transport system that raises immune response towards AIDS-causing viruses. CMV has been engineered to express HIV or SIV proteins.

CMV offers protection for life and people who carry CMV have few or no symptoms. This virus, according to the researchers could enable to immune system to target HIV and destroy the virus in the body.

The study was published in the journal Nature Medicine.